Ash dieback disease

What is ash dieback?

Ash dieback is caused by a fungal organism called Hymenoscyphus fraxineus. The disease causes leaf loss and crown dieback in affected trees and can lead to tree death. The fungal disease is carried on the wind and by transportation of infected trees.

The disease was first found in the UK in early 2012 on young ash trees in tree nurseries and recently planted sites. Initial sightings were scattered around the country with most being recorded in the South East and East of the UK.

Which ash trees are at risk?

All ash trees are vulnerable; it is not known to spread to any other type of tree. It is particularly destructive to young trees. Older trees can survive initial attacks, but can succumb eventually after several seasons of infection

The disease does not cause rapid or catastrophic failure of trees. Any danger from dead or dying trees is likely to be gradual and obvious over a period of year. Hymenoscyphus fraxineus is not fatal to all ash trees. The older infected trees may be able to survive indefinitely, although they may be at increased risk of other infections.

For more information, visit the Forestry Commission website.

How serious is ash dieback?

The disease is a potentially serious threat to ash trees across the UK.

The disease has already caused extensive loss of ash trees in mainland Europe and could be a major threat to wild and planted ash in the UK if it takes hold here.

The disease has spread to most areas, including Dorset and the South West. Current estimates put the percentage of infected ash trees in Dorset at about 90%.

There is a government ban on the import of ash plants into the UK and on the movement of ash plants, seeds and trees into and around the UK. The ban does not extend to the movement of ash timber or firewood except from sites where the disease has already been found.

What are the signs of the disease?

The disease is not always easy to identify and it is sometimes mistaken for Ash canker (caused by bacterium Pseudomonas syringae). If in doubt, check Forestry Commission for guides that explain the symptoms and identification of the disease.

It is characterised by the premature loss of leaves from the outer parts of the crown (top and sides), accompanied by long diamond-shaped lesions or areas of sunken and discoloured bark on twigs.

These lesions girdle twigs and small branches, starving the leaves above of water and nutrients and causing whole branches to die. In mature trees, it is the new growth that is affected.

You can get more information on the Food and Environment Research Agencys (FERA) video which shows how to identify ash dieback disease.

See also the guide to the symptoms of Ash dieback disease.

Because one of the symptoms of Hymenoscyphus fraxineus is leaf loss, identification of the disease will be difficult in the autumn when the trees are losing their leaves.

What should I do if I think I have found Ash dieback disease disease?

The disease has been classified as 'notifiable' by DEFRA, which means that any suspected cases of the disease must be reported to the appropriate plant health authorities.

If you think that you have identified Hymenoscyphus fraxineus on an ash tree, then you should contact one of the following bodies:

• Forest Research Disease Diagnostic Advisory Service
• Forestry Commission Plant Health Service